A quantitative framework for demographic trends in size-structured populations: analysis of threats to floodplain forests

Studies of population dynamics are continually seeking to develop quantitative approaches that can be easily applied to widely available data in ways that can guide management decisions. We present a method for quantifying demographic trends in size-structured populations that we applied to forest tree species and changes in forest structure associated with different threats to help identify forest health priorities. Strengths of the approach are that tree size and growth rate can be controlled for to separate mortality impacts of particular threats from background rates associated with stand self-thinning. We illustrate the method with tree census data from Connecticut River floodplain forests. We found that these floodplain forests are currently declining demographically across all sizes, with floodplain pioneer tree species particularly affected. Cutting by a large beaver population is contributing to this decline. Specifically, beavers are cutting 11.4% of the Salix nigra and 1.6% of the Populus deltoides trees annually. We also showed quantitatively that Dutch elm disease and invasive lianas are important threats to the health of these forests. We estimated that Dutch elm disease caused at least 9.5% of all tree mortality. Invasive Celastrus orbiculatus lianas were implicated in 9.8% of the mortality of large floodplain trees (i.e., DBH = 60 cm) on the Lower Connecticut River (i.e., GDD > 3463, base 0°C). Overall, we found that the method is flexible and could be applied to a wide range of forest types and threats.